i would wear a summer dress or something similar you feel comfortable in with a cute pair of sandals. you could always go for a natural look (since he likes the way you look naturally) but still wear a little makeup too! there are a lot of online tutorials for "no makeup" looks. these two by michelle money are really similar but two of my favorites!
According to me, if you are going to a park for date, wear a nice pair of jeans with top. You can add accessories like scarf, belt, bracelet, etc. All the best.
A summer dress would look cute. And for makeup, keep it simple:) just do some eyeliner, mascara, blush, and some concealer/foundation
For the first date I prefer you to touch up with light makeup through your mate came to feel the natural beauty of yours. Dressing well is more important than anything else. Dressing something like sporty and youth iconic. You can add winter coats into your great looks as well.